The goal of this proposal is to understand the mechanism of RNA interference (RNAi), a type homologydependent, post-transcriptional gene silencing that is triggered by double-stranded RNA, in C. elegans. A genetic screen will be performed to isolate mutants that are resistant to RNAi. Mutants that are involved in the initiation step of RNAi are of particular interest and will be mapped and cloned. Proteins that interact with the previously identified initiators of RNAi will be isolated using yeast two-hybrid methods, and the function of these proteins in RNAi will be assessed. The identity of factors involved in the initiation of RNAi will lead to a better understanding of how RNAi is triggered in C. elegans. Knowing the identity of genes required for RNAi in C. elegans may reveal an orthologous pathway in humans as the genome sequence unfolds. These studies may help us to harness this process as a novel therapeutic strategy in the treatment of genetic disorders. Finally, RNAi is hypothesized to be part of a natural defense mechanism that limits the mobility of transposable elements, and similar mechanisms of transposon silencing may exist in mammalian systems.